The invention relates to a method for surface treating a substrate and a device for carrying out the method.
It is known from EP 0 279 371 B1 to apply an aerosol to the surface of a plastic material to attain an improved and more uniform adhesion for a subsequent coating or, with suitable modification of the aerosols, to directly coat the surface of the plastic material.
In the first case, for example water can be atomized to form aerosols, whereas in the second case, i.e. with direct coating of the plastic surface, suitable solutions are used.
For producing the corona discharge, two spaced-apart electrodes are required, wherein a high voltage is applied to at least one of the electrodes, while the other electrode is connected to ground. The aerosols are sprayed into the discharge region, which is defined by an effective gap formed between a corona electrode and the substrate and having a width of about ≦2 mm. The corona treatment itself is known in the art, for example from the aforementioned EP 0 279 371 B1, so that expressively reference is made thereto.
Due to their structure, the atomizers used for producing the aerosols are not capable to produce aerosols having a uniform size, i.e. aerosols of different size are produced. It has been observed that this diminishes the surface treatment of the substrate, both with respect to producing adhesion forces, by forming adhesion nests of different strength, as well as with respect to applying a coating, when the aerosols are provided for coating the surface of the substrate directly.
It should be noted in this context that the aerosols are atomized from above, in the direction of the force of gravity, to the substrate below, as well as the corona discharge itself, by a corresponding arrangement of the spray nozzles associated with the atomizer or the electrodes.
A method for surface treatment of a substrate with an electric corona discharge is discussed in EP 0 160 889 A2 related to a different technical field, wherein a fluid operates as the corona electrode, wherein the fluid exits from an electrode profile under pressure and advantageously has a good electrical conductivity, wherein the electrode profile is arranged at the bottom with respect to a support electrode.
In contrast to a method in the same technical field, where an effective gap is formed between the corona electrode and the substrate, the conventional type of fluid electrode has no gap to the substrate, i.e. this electrode contacts the substrate directly.
To obtain a corona discharge, a continuous supply of fluid is required which should also have good electrical conductivity. I.e., a permanent fluid wall must be placed between the electrode profile from which the fluid exits and the substrate, so as to cause the corona discharge.
However, this method known from the published literature is not practical for several reasons. For example, parts of the device are permanently connected to an electric potential, which is not acceptable with respect to the occupational safety. Although it is recommended to produce the support electrode from an electrically insulating material and to electrically insulate the supply line for the fluid from a reservoir, this reservoir however must be filled, which is life-threatening during continuous operation of the facility. In this case, it is recommended to interrupt the production flow and to switch the entire device off, which makes operating the device and carrying out this method uneconomical.
It should also be considered in this context that fluid adhering to the substrate cannot be dried off in the desired time. Moreover, the applied fluid does not completely adhere on the substrate, thus requiring suitable capture measures for the draining fluid. Nevertheless, contamination of components by the draining fluid cannot be prevented, thus requiring frequent cleaning and possibly damaging these components.